(1) Field of the Invention
This invention relates to a bonded wafer consisting of two wafers joined together, and to a method of making such bonded wafers.
(2) Description of the Prior Art
As a method for forming a single crystal semiconductor film over a dielectric substrate, there has been known, of old, a technology which is characterized by epitaxially growing a single crystal silicon film or the like over a single crystal sapphire substrate. In this technology, however, because there exists a disagreement in lattice constant between the dielectric substrate and the silicon single crystal which is grown in vapor phase, numerous crystal defects occur in the silicon vapor-phase grown layer, and for this reason this technology is of no practical use.
Another conventional technology of providing a single crystal film on a silicon substrate is characterized by first forming a thermally oxidized oxide film across the surface of the silicon substrate, laying a polycrystal or amorphous silicon film over this oxide film, and applying to this an energy beam such as an electron beam or a laser beam in a manner such that the beam spot thereon is shifted along lines all in one and the same direction to thereby melt the silicon film, and then cooling and solidifying the film to obtain a monolithic single crystal film.
Now, the technology of changing the silicon polycrystal film over the oxide film into the single crystal film by means of the laser beam or the like is disclosed, for example, in Japanese Patent Kokoku 62-34716. According to this publication, a single crystal projection is monolithically formed at a corner of the periphery of a single crystal silicon substrate, and with this projection as the seed crystal, single crystallization of the polycrystal film is effected. Although locallized growth of a single crystal is possible owing to an interaction between the single crystal projection and the molten silicon oxide film, it is difficult to obtain a silicon single crystal film of practical use.
Under the circumstances, bonded wafers having a Si-on-insulation structure (SOI structure) have come to draw attention of scientists in the field. An SOI bonded wafer is made from two semiconductor silicon mirror wafers in the following manner. (Incidentally, a mirror wafer is a wafer at least one of whose two faces has specular gloss.) At least one of the wafers is subjected to an oxidation treatment so that an oxide film is formed over at least one face of the wafer which face has specular gloss. Then, the two wafers are joined together with the specular-glossy faces meeting each other; hence the oxide layer is sandwiched between the wafers. The joined wafers are heated at an appropriate temperature until sufficient bonding strength is created between the wafers. The external face of at least one of the wafers is ground, and thereafter polished until it becomes a thin film. This wafer is called "the bond wafer". The other wafer incidentally is called "the base wafer".